6 Answers
6

Totally depends on your goal. Think background first. What story do you want to tell? Epic background, big mountains. Looking to deliver a sense of grandeur with your subject. Go big! f/22 or higher if you have it. If you want to really isolate your subject and use the background as simple tone, a splash of delightful color, open up to f/1.4. Be careful here as you may have parts of the face go out of focus. Step back with a long lens and you'll eliminate that problem (by increasing the over all dof).

Are you sure? Longer lenses usually have thinner DoF, wider angles have deeper. I agree with your immediate statement about the background, the role it plays is important, but after that it doesn't appear to be accurate. I should note that I mean apparent depth of field in regards to the focal length.
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John Cavan♦Jan 6 '11 at 3:25

Tough question. Not sure that there's going to be a 'right' answer not because it varies from shot to shot, but more that it varies from intention to intention. Do I want a background more in focus (great clouds and sunset along with an engaged couple in the foreground, for example)? Then I'm bumping up to f/8 or higher... Nondescript or uninteresting background? Then I'm shooting to keep only the people in focus while the background blows out to a pretty, out-of-focus backdrop? I'm shooting as close to f/2.8 as I can get away with... It all varies based on my intention with each shot, and there isn't a real 'rule of thumb' that I can apply...

I usually shoot portraits of family and friends using the 70-200/2.8 at f/2.8->f/3.5. My portraiture style tends to be head shots filling the frame, so I found out pretty quickly that my 50/1.4 is just too wide (meaning, too narrow a DoF) for this style. Some part of the face will always be out of focus when shooting wider than f/2.8.

I'm puzzled by the comparison. At most distances, the DoF for any given image size and any given f-stop will be the same no matter what focal length you use. (E.g., the DoF is 4 inches at f/2.8 for a subject shot at 5 feet with a 50 mm lens, 10 feet with a 100, or 20 feet with a 200 and in all cases the subject is the same size on the image.) It seems you are mainly opting to get further from your subjects, because of course you could always shoot the 50 mm at f/2.8 to increase the DoF.
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whuberJan 5 '11 at 23:47

Well, the reason is probably b/c (for me) the 70-200 is much more versatile than the 50 that I just use it at f/2.8 instead of stopping down the 50 to f/2.8.
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ysapJan 6 '11 at 0:17

Keep in mind that the background blur is as much a function of your focal distance as it is aperture. With my 50mm f/1.4 lens, I'll set my aperture to approx f/2.8-5.6 (even though I can take it down to 1.4) - this will maximize the sharpness of my subject while still leaving a pronounced separation between the subject and the background. With a 50mm lens on this setting on my cropped-sensor camera, I'll position myself about 2-3 meters from the subject. Taking it down to 1.4 will make the picture look even more gorgeous for a 4x6" print, but prints larger than that the softness will become very apparent.

On my zoom lens, I usually zoom in all the way to 85mm and keep the aperture around f/5.6.

One issue with some small apertures mentioned here is that they will get you to the diffraction limit of your lens/sensor combination. The general observation is that most lenses are soft wide open (due to imperfections) and become soft again past f/11 or so (due to diffraction). In general lenses are best one or so stop down from wide open.

As mentioned before in the thread (and as you can see from experimenting with the DoF calculators) for a 50mm lens even at f/4 you get nice shallow DoF at 1m or so. At 2m f/4 will let you have the whole head in focus but isolate your subject from foreground and background.

I tend to shoot around f/3.0 or so, indoors or out. If there's a really cool background, I'll go up to f/10, but I find I don't like to shoot much in the f/22 range, no matter the target... It does depend a bit if I'm wanting to isolate the background, but I find that I sometimes struggle a bit with wanting to include it or isolate it.